Anatomy of Solenostelic Ferns . 731 
I have no doubt that these also represent metamorphozed 
sieve-tubes. 
Attention must finally be drawn to the very remarkable 
mucilage ducts or vessels that are to be found in the external 
phloem of the petiolar strands of Dicksonia Barometz . They 
occur in the metaphloem, and are easily distinguished from 
the rest of its elements by their greater size and their dense, 
deeply staining mucilaginous contents. The structure of 
these elements was not investigated in detail, but it appears 
that they are formed by a number of elongated cells arranged 
in vertical series, the walls of which have become reabsorbed 
at certain points where they are contiguous, so that their con- 
tents are continuous throughout the series. In position they 
correspond to the inmost sieve-tubes of the metaphloem, but 
the perforations appear to be true perforations, and they 
are much too wide to resemble the sieve-plates. Whether 
they are to be regarded as metamorphozed sieve-tubes or not 
was not decided. From a figure given by Bertrand and 
Cornaille (loc. cit., p. 58, Fig. 30) similar structures appear 
to be present in the petiole of Dicksonia regalis. 
The Systematic Value of the Vascular Anatomy. 
No really satisfactory conclusion upon the degree of impor- 
tance that ought to be assigned to the vascular anatomy as 
a factor in the classification of the Polypodiaceae can be arrived 
at until the structure of at least a majority of the species 
of the various genera has been correctly described. The 
facts already at our disposal, although not so extensive as one 
might wish, are still sufficient to show that the characters 
brought to light by the study of the vascular anatomy will 
probably prove to be of great assistance in constructing a 
natural classification, and they must certainly be taken into 
account by the systematists. The anatomy of so many 
species, even in the genera especially dealt with in this paper, 
is still unknown, that the tentative nature of any conclusions 
drawn from the results obtained must, in the first place, be 
thoroughly understood. 
